This invention relates to video signal processing and more specifically to converting video signals from a format (such as RGB) having sequential scanning to an interlaced scanning format as used in composite video (television).
As well known, the video signal format in a computer is not directly compatible with the video signal format used in composite video, e.g. television. Thus it is not possible to directly couple computer video signals into a television set or other television-standard video device, even though it is advantageous to convert computer video signals into signals that may be used for recording to a video cassette recorder or for a display on a television monitor, for instance a large screen monitor.
A personal computer (PC) typically outputs red, green, blue (RGB) analog video signals in a progressive scanning format. These RGB signals are typically the type generated by a VGA controller in the personal computer for driving a PC monitor. In contrast, a television device accepts video input in composite video (or sometimes S-VHS video) format which uses interlaced scanning, whereby in each video field only every other line is scanned, and two complete video fields are one frame. Thus it is known to be useful to convert computer images or graphics data for instance in the RGB format into an analog television NTSC or PAL or S-VHS standard signal. One such converter is described in Zhang et al., U.S. Pat. No. 5,526,055 issued Jun. 11, 1996. (U.S. Pat. No. 5,526,055 is not acknowledged herein as prior art to the present application, but is exemplary of video converters.) See also U.S. Pat. No. 5,455,628, issued Oct. 3, 1995, to Bishop, incorporated herein by reference, also disclosing such a converter.
However, the present inventors have determined that the performance of known video converters is inadequate in terms of video signal processing, and improvements are needed to provide a better output television image.
In accordance with this invention, an RGB to composite video converter includes a flicker filter to essentially eliminate the flicker in the luminance component of the composite video, which otherwise would appear in the television image. The flicker filter accepts input signals in the YCbCr format and eliminates flickering by averaging the pixels, with minimum memory component requirements. The flicker filter incorporates a user control to perform a three line, two line or no filtering.
The color subcarrier signal, which is needed in such a converter to convey the color information, is itself generated by a digital frequency synthesizer.
Each of the digitized RGB signals is generated by a digital to analog converter from a digital signal source in a personal computer. Its waveforms are staircase-like and its level transitions along with the digital to analog converter clock signal in the PC. The analog to digital converters in the present video converter include a clock phase adjustment feature to provide optimum sampling of the RGB input video signals. Adjusting the phase of the analog to digital converters"" clock signal enables the input signal to be sampled when its input level is not in transition, so as to properly convert the signals into digital form.